Passive intermodulation (PIM) occurs when signals are present in a passive device that exhibits some non-linear behavior. In a base station, such as an evolved node B (eNB) of a long term evolution (LTE) wireless communication system, high power signals can cause measureable PIM due to non-linear behavior of components such as RF transmission cables, duplexers, connectors, antenna, or some object external to an antenna of the base station.
Passive intermodulation (PIM) distortion is problematic for multi-band transmitters when intermodulation distortion (IMD) falls into occupied receiver (Rx) channels, which desensitizes the receiver. Multi-band signals are an important characteristic of LTE and multi-standard radios, so the occurrence of PIM problems will be more frequent than with previous cellular wireless communication standards. There are many causes of PIM related to materials, manufacturing quality and quality of workmanship by installation and maintenance technicians.
A test can be conducted during the installation of a base station to detect sources of PIM. In such a PIM test, one technician monitors the third order intermodulation distortion (IMD3) of two high power input signals, while another person taps and wiggles (dynamic testing) all connectors, cables and components between the TX port on the radio and the antenna. Failed PIM tests could be a result of failing components, over or under-tightened connectors, or dirty connectors, including metal flakes. Some PIM sources will not be detected during this test and are later catalyzed by some change in the environment, e.g., wind, train vibrations, temperature, etc. Other PIM sources can develop with aging, such as corrosion. If a PIM problem is detected at a base station during normal operation, then a crew is sent to identify and remove the PIM source. In some cases the PIM source may be due to some object external to the base station like a fence. In this case the PIM source cannot be removed, so the antenna may need to be re-oriented or the transmit power decreased.
In a base station, the high power transmit signal of the base station transmitter is typically a source of PIM distortion when the transmit signal passes through a passive device that exhibits some non-linearity. Therefore, the PIM distortion may be considered a nonlinear function of the transmit signal. Several methods have been proposed to estimate PIM distortion that falls into a band of the desired receive signal. One such method involves multiplying transmitted signals to generate 2nd and higher order IMD products. Complex weights are then applied to each IMD product in a manner that reduces the PIM distortion in the receiver pass band. This approach models the PIM source with a polynomial model, and passes the transmit signal through the polynomial model to estimate the PIM distortion in the receiver pass band.
In another approach, an output signal of the power amplifier (PA) of the transmitter is tapped and fed to an auxiliary receiver, called a cancellation receiver. The cancellation receiver is tuned to the desired receiver pass band. The signal content in the receiver pass band at the PA output is presumed to be due to intermodulation products (IMPs) from the transmit signals caused by non-linear behavior of the PA. These IMPs in the receiver pass band are then adaptively filtered in the cancellation receiver so as to match the PIM signal at the output of the main receiver. The estimate of the PIM in the cancellation receiver is subtracted from the output of the main receiver.
A problem with these two approaches is that their models are trained with a PIM signal that is typically weaker than the uplink signal, even when the uplink signal is only noise and in-band interference. Further, both of these approaches require provision of an additional receiver.